Tech Stuff

Ford distributorless ignition system

These spark plugs are arranged in order as they were removed from a V6 engine with DIS ignition. Note how the center electrodes are worn down on all three plugs from one bank. Can you determine which way the current flows across the gaps?
<p>These spark plugs are arranged in order as they were removed from a V6 engine with DIS ignition. Note how the center electrodes are worn down on all three plugs from one bank. Can you determine which way the current flows across the gaps?</p>

Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer. He began his writing career writing service manuals at Chilton Book Co. He currently holds ASE Master Technician and L1 certifications and has participated in ASE test writing workshops.

The distributorless ignition system (DIS) was introduced in the 1980s to eliminate parts that affect emissions as they wear, but it also helped improve performance and fuel mileage. Today DIS has been phased out in favor of coil-on-plug (COP) ignition, but Ford was still producing engines with DIS as late as 2008, so you’ll be servicing these systems for many years to come.

Like COP ignition, a distributorless ignition system consists of ignition coils, the crankshaft and camshaft position sensors and the control circuits. Earlier systems have some kind of ignition control module that operates the coil packs and communicates with the engine’s powertrain control module (PCM), but later systems integrated all controls into the PCM. The most prominent feature of any distributorless ignition system is that each coil fires the spark plug for two different cylinders, both at the same time.

Of all the various ignition coil designs, a DIS coil is unique. To understand why, we need to review some basics first. You probably already know that a coil is actually two windings (coils) of wire.

The primary winding is a large-diameter wire wound around a core of ferrous metal, typically a stack of steel plates cut to a specific shape. Around the outside of that primary winding is the secondary winding. This is much finer wire with about 100 times the number of windings in the primary.

In other ignition coils, both windings are connected together at the power source (ignition +). The primary connects to ground through the module that operates the ignition system. The secondary connects to ground through the body of the spark plug, but only when the voltage is high enough to jump the gap to its ground electrode.

In a DIS coil, the secondary winding is not connected to the primary winding or to the power source. Both ends of the secondary winding are connected to a spark plug. To understand how current can flow through a circuit that’s not connected to a power source, we need to remember how a coil works.

Cracked spark plug insulation creates a perfect opportunity for carbon tracking.
<p>Cracked spark plug insulation creates a perfect opportunity for carbon tracking.</p>

The coil is a transformer

When a coil of wire is surrounded by a magnetic field that’s moving, voltage is generated (induced) in that wire. If the wire is part of a complete circuit, current will flow. An ignition coil creates a stationary magnetic field when current flows through the primary winding.

When the primary current is switched off, the magnetic field collapses (moves), and this induces voltage in the secondary winding. Since there are more turns of wire in the secondary winding, it acts as a transformer to boost the initial (battery) voltage up to 20,000 volts or more depending on the demand (we’ll get to that later).

That voltage can’t go anywhere unless there’s a complete circuit. In a normal coil the spark plug is the secondary winding’s ground and the other end of the winding is connected to the battery, but that’s just to make a complete circuit: The voltage is generated by induction.

In a DIS coil, both ends of the secondary winding are connected to a spark plug. When the voltage is high enough to jump both gaps, current will flow through that circuit.

Tags: Spark plugs 
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